Vehicle evaporative emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of undesired evaporative emissions that could release fuel vapors to the atmosphere.
Undesired evaporative emissions may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system and evaporative emissions control system may be isolated at an engine-off event. The pressure in such a fuel system and evaporative emissions control system will increase if the tank is heated further (e.g., from hot exhaust or a hot parking surface) as liquid fuel vaporizes. If the pressure rise meets or exceeds a predetermined threshold, it may be indicated that the fuel system and the evaporative emissions control system are free from undesired evaporative emissions. Alternatively, if during the pressure rise portion of the test the pressure curve reaches a zero-slope prior to reaching the threshold, as fuel in the fuel tank cools, a vacuum is generated in the fuel system and evaporative emissions system as fuel vapors condense to liquid fuel. Vacuum generation is monitored and undesired emissions identified based on expected vacuum development or expected rates of vacuum development. The EONV test may be monitored for a period of time based on available battery charge.
However, the EONV test is prone to false failures based on customer driving and parking habits. For example, a refueling event that fills the fuel tank with relatively cool liquid fuel followed by a short ensuing trip may fail to heat the fuel bulk mass and may result in a false fail if an EONV test is run. Further, the rates of pressure build and vacuum development are based in part on the ambient temperature. During mild weather conditions, the ambient temperature may restrict the amount of heating or cooling of the fuel tank following engine shut-off, and thus limit the rate of pressure or vacuum development. As such, in a case wherein a pressure build does not reach the expected threshold, the subsequent vacuum build may additionally not reach expected threshold level in the time allotted for the EONV test based on available battery charge. This may result in a false-fail condition, leading to potentially unnecessary engine service. Given the above-described issues with relying on EONV tests to diagnose vehicle fuel system and/or evaporative emissions systems, alternate approaches have been developed.
In one example, US Patent Application 2015/0090006 A1 teaches conducting leak detection in an evaporative emission systems control system by using a pump configured to both pressurize and evacuate the fuel system. However, the inventors herein have recognized potential issues with such a method. For example, the use of an external pump introduces additional costs, occupies additional space in the vehicle, and includes the potential for malfunction.
In another example, US Patent Application 20140074385 teaches, during engine-off conditions, operating a fuel pump coupled to the fuel tank to initiate an evaporative emissions test. By operating the fuel pump, fuel in the fuel tank is agitated, causing a fuel vapor pressure to increase. Following the fuel tank pressure build-up, pump operation is discontinued, and a rate of pressure decay or bleed-down is monitored and compared to a threshold rate. However, the inventors have additionally recognized a potential issue with such a method. For example, increasing fuel vapor pressure may in some examples result in fuel vapors being routed to a fuel vapor canister positioned in the evaporative emissions system. Fuel vapors routed to the canister may increase the loading state of the canister, which may in some cases lead to an increase in bleed emissions from the canister depending on the adsorbent capacity of the canister.
In yet another example, U.S. Pat. No. 9,140,627 teaches, during a vacuum portion of an EONV test, operating a cooling fan to increase a fuel system vacuum, and indicating the presence or absence of undesired evaporative emissions based on the increased vacuum. However, the inventors have further recognized a potential issue with such a method. For example, while the method may serve to facilitate an increased level of vacuum during the vacuum build portion of an EONV test, the portion of the EONV test wherein a pressure build is monitored is not able to be manipulated by such a method.
The inventors herein have recognized the above issues, and developed systems and methods to at least partially address the problems. One example method for an engine comprises: adjusting pressure in a vehicle evaporative emissions system by raising or lowering a vehicle body element; and conducting a test in the evaporative emissions control system for detection of evaporative emissions based on the adjusted pressure. In this way, by generating vacuum and/or pressure in the evaporative emissions control system using body component lifting, for example through lift gate cylinders, undesired evaporative emissions may be detected.
In one example, the vehicle system may comprise a body element such as a hood, a trunk, and/or a gull wing style door which may be raised and lowered based on operator demand. Pneumatic or screw motor cylinders may be present in the vehicle lift gate which may be used to draw in ambient air via an orifice. During raising of the hood or trunk, a canister vent valve of an engine evaporative emissions control system may be closed and a lift gate valve coupling the body element to the evaporative emissions control system may be opened such that the lift gate cylinders may draw out air from the fuel vapor system, creating a vacuum therein. Once a sufficient level of vacuum has built in the fuel vapor system, the fuel vapor system may be sealed and bleed-down of vacuum may be observed over a defined period of time. If the rate of vacuum decay is higher than an expected rate, it may be inferred that undesired evaporative emissions may be present. If the vehicle hood or trunk is lowered during the test, the lift gate valve may have to be re-opened, the current diagnostic test may be terminated, and the fuel vapor system may be vented. Along with the lift gate valve, the canister vent valve may be opened to facilitate the venting process and may be subsequently closed within a short time while the hood or trunk is still in the process of being lowered. The closing of the lift gate valve and the opening and closing of the canister vent valve may cause air to be compressed into the fuel vapor system, thereby creating a higher pressure in the fuel vapor system. The higher pressure may then be used for a further diagnostic routine to detect any undesired emissions in the evaporative emissions control system. If the rate of higher pressure decay is higher than expected, it may be inferred undesired evaporative emissions are present in the system.
In this way, a cylinder coupled to a vehicle body element such as a hood, or a trunk may be used to apply vacuum and/or positive pressure in a fuel vapor system. By using this vacuum and/or higher pressure generated by the lift gate cylinders, detection of undesired emissions in the evaporative emissions control system may be opportunistically carried out without dependence on ambient temperature and duration of engine key-off. In addition, the need of dedicated vacuum/positive pressure pumps for providing the pressure is reduced. The technical effect of using lift gate cylinders coupled to a vehicle hood or trunk for evaporative emissions control system diagnostic test is that the test may be carried out both during raising of the hood or trunk (using the vacuum generated in the process) and during lowering of the hood or trunk (using the positive pressure generated in the process). By coupling the lift gate cylinders directly to a vent line of the fuel vapor system, a time required for attaining the desired vacuum or higher pressure may be lowered and an evaporative emissions control system diagnostic test may be carried out within a short time of engine operation, such as in the limited engine-on time of a hybrid vehicle. As a result, a larger number of diagnostics tests may be carried out within a drive cycle, improving the completion ratio for the evaporative emissions control system monitoring. Therefore, by using a lift gate cylinder for evaporative emissions control system diagnostics, engine emissions compliance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.